Introduction to
Macromolecules
Explore the four major biological macromolecules that form the foundation of all living systems: their structure, function, and the reactions that build them.
AP Biology
What Are Macromolecules?
Macromolecules are large, complex molecules built from smaller subunits called monomers. When monomers join together, they form long chains called polymers. These giant molecules carry out the essential functions of life: storing energy, providing structure, catalyzing reactions, and encoding genetic information.
Monomers and Polymers
Monomer
A single, small molecule that serves as a building block
Polymer
A long chain of repeating monomers linked by covalent bonds
Dehydration Synthesis vs Hydrolysis
The Four Classes of Macromolecules
Carbohydrates
Quick energy and structural support. Monomers: monosaccharides
Lipids
Long-term energy, membranes, signaling. Not true polymers
Proteins
Enzymes, structure, transport. Monomers: amino acids
Nucleic Acids
Genetic information storage and transfer. Monomers: nucleotides
Carbohydrates: Structure and Function
Lipids: Not True Polymers
Saturated vs Unsaturated Fats
| Saturated | Unsaturated | |
|---|---|---|
| Carbon bonds | All single bonds | One or more double bonds |
| Shape | Straight chains: pack tightly | Kinked chains: cannot pack |
| State at room temp | Solid (butter, lard) | Liquid (olive oil) |
| Source | Mostly animal fats | Mostly plant and fish oils |
Proteins: The Workhorses of the Cell
Four Levels of Protein Structure
Primary
Linear sequence of amino acids in the polypeptide chain
Secondary
Local folding into alpha helices and beta pleated sheets via H-bonds
Tertiary
Overall 3D shape from R group interactions (hydrophobic, ionic, disulfide)
Quaternary
Multiple polypeptide subunits combine (e.g. hemoglobin has 4 subunits)
Nucleic Acids: The Information Molecules
Energy Storage: Carbs vs Lipids
| Carbohydrates | Lipids | |
|---|---|---|
| Energy per gram | 4 kcal/g | 9 kcal/g |
| Access speed | Fast: quick energy source | Slow: requires more processing |
| Storage form | Glycogen (animals), Starch (plants) | Adipose tissue (body fat) |
| Water solubility | Soluble (hydrophilic) | Insoluble (hydrophobic) |
| Best for | Short-term energy | Long-term energy storage |
Glycogen and fat store the same energy in different physical packages
Using approximate nutritional energy densities of 4 and 9 kcal/g, 100 kcal requires 25 g of dry glycogen or 11.1 g of fat. If glycogen carries 2 g of water per gram, its total storage mass is 75 g; hydration varies.Compare the same energy target
Using approximate nutritional energy densities of 4 and 9 kcal/g, 100 kcal requires 25 g of dry glycogen or 11.1 g of fat. If glycogen carries 2 g of water per gram, its total storage mass is 75 g; hydration varies.How to Identify a Macromolecule
1
Check the elements. All macromolecules contain C, H, and O. Proteins also contain N and S. Nucleic acids contain N and P.
2
Identify the monomer. Monosaccharides signal carbohydrates. Amino acids signal proteins. Nucleotides signal nucleic acids.
3
Look at the bond type. Glycosidic bonds connect sugars. Peptide bonds connect amino acids. Phosphodiester bonds connect nucleotides. Ester bonds link fatty acids to glycerol.
4
Consider the function. Quick energy = carbohydrate. Long-term energy/membranes = lipid. Catalysis/structure = protein. Genetic info = nucleic acid.
Example: Identifying Glycogen
A molecule is found to be a highly branched polymer of glucose stored in liver and muscle cells. What type of macromolecule is it?
1
Monomer: Glucose is a monosaccharide, so this is a carbohydrate.
2
Bond type: Glucose monomers are linked by glycosidic bonds via dehydration synthesis.
3
Function: Stored in liver and muscle for quick energy access: this is glycogen, the animal storage polysaccharide.
Example: Why Shape Matters in Proteins
Example: The Phospholipid Bilayer
Common Mistakes to Avoid
✕ Lipids are polymers
Lipids are not true polymers. They are not built from repeating monomer subunits linked by polymerization.
✕ All fats are bad
Unsaturated fats (olive oil, fish oil) are essential for health. Only trans fats and excess saturated fats are problematic.
✕ Denaturation = destruction
Denaturation changes a protein's shape, not its primary amino acid sequence. Sometimes denaturation is reversible.
Summary: All Four Macromolecules
| Carbs | Lipids | Proteins | Nucleic Acids | |
|---|---|---|---|---|
| Monomer | Monosaccharide | Glycerol + fatty acids | Amino acid | Nucleotide |
| Bond | Glycosidic | Ester | Peptide | Phosphodiester |
| Elements | C, H, O | C, H, O | C, H, O, N, S | C, H, O, N, P |
| Key function | Quick energy | Long-term energy, membranes | Enzymes, structure | Genetic info |
| Example | Starch, Cellulose | Triglycerides | Hemoglobin | DNA, RNA |
Question 1 of 5
Which reaction builds polymers by removing a water molecule?
Dehydration synthesis (also called condensation) joins monomers by removing \( \text{H}_2\text{O} \). Hydrolysis is the reverse: it adds water to break bonds.
Question 2 of 5
A molecule contains C, H, O, N, and P. Which macromolecule class does it most likely belong to?
Nucleic acids (DNA and RNA) are the only macromolecule class that contains phosphorus (P). Proteins contain N and S, but not P. Carbohydrates and lipids contain only C, H, and O.
Question 3 of 5
What type of bond links amino acids together in a protein?
A peptide bond forms between the amino group of one amino acid and the carboxyl group of another via dehydration synthesis. Glycosidic bonds link sugars, phosphodiester bonds link nucleotides, and ester bonds link fatty acids to glycerol.
Question 4 of 5
Why are lipids not considered true polymers?
Unlike carbohydrates, proteins, and nucleic acids, lipids are not assembled by linking identical repeating monomers through polymerization. A triglyceride, for example, is made from one glycerol and three fatty acids: different components, not a repeating chain.
Question 5 of 5
An enzyme loses its ability to catalyze a reaction after being exposed to extreme heat. What has occurred?
Denaturation disrupts the higher-level (secondary, tertiary, quaternary) structure of a protein, altering the shape of its active site. The primary structure (amino acid sequence) remains intact. The enzyme simply cannot bind its substrate anymore.